In projection radiography the detected radiation field contains both primary and secondary (scattered) components. Scattered X-rays reduce the contrast of images. Antiscatter grids absorb scattered radiation and increase image contrast. Devices that hold grids and X-ray film are known as “Bucky” from the name of Gustav Bucky, who first designed stationary grids in 1913 in Germany, and patented them in the USA, U.S. Pat. No. 1,164,987, issued Dec. 21, 1915. At the present time both stationary and moving grids are used for this purpose. The disadvantage of stationary grids is that they create line artifacts on film and digital images and, when changed in size for display or reprinting, create visible Moire patterns due to spatial aliasing. Moving grids effectively remove the stripe artifacts by means of temporal blur. Of the various stationary grid designs, including parallel, focused, and crossed, the most commonly used are linear grids with parallel stripes in one direction. Crossed grids have grid stripes in both orthogonal directions and are used less frequently. The most important features of a linear grid are its resolution in line/mm (millimeter) and the ratio between grid stripes height and distance. Due to the spatial regularity of the grids in one direction, strengthened by geometrical cut-off effect, the stripe artifacts in digital radiographic images can be considered as 1-D coherent spatial noise of a frequency corresponding to the distance between stripes.
In U.S. Pat. No. 5,661,818, issued Aug. 26, 1997, inventors Gaborski et. al., discloses a grid artifacts detection method which is based on a double auto-correlation calculation. Variances are measured independently, both horizontally and vertically and a statistical F test is performed to determine if the variances are the same over a randomly chosen sampling of locations within the image. Votes are then tallied and if a majority indicates that the variances are different, a decision is made in favor of a grid being present. This method, however, does not provide a key parameter—the grid line frequency—as well as other important grid features that might be useful in further suppression. Another method was proposed in U.S. Pat. No. 6,269,176, issued Jul. 31, 2001, inventors Barski et al. It is based on 1-D spectral approach and comprises several steps: finding a window for analysis, obtaining averaged 1-D Fourier power spectra in each direction, searching for the maxima in the right part of smoothed and morphologically filtered averaged spectra as grid peak candidates, collecting the attributes of each peak, sorting the candidates in terms of figure-of-merit (FOM), and picking the best candidate with the highest FOM. This method provides such grid peak attributes as orientation, frequency, magnitude, total energy, half-width of full maximum, coherency. Although these methods were suitable for the uses for which they were intended, the main disadvantage of both of these methods is the uncertainty in picking the right candidate if several frequencies are detected, due to the lack of dynamic tracking of success in the 1-D analysis. A second disadvantage is that neither method provides information about signal-to-noise ratio (SNR) in the frequency grid peak area, which is needed as a parameter for further automatic tuning and design of the attenuation filter.
In the present invention a method for grid linear artifact detection is proposed based on 2-D dynamic correlation in both spatial and frequency domains. This method provides results including grid orientation, frequency, and SNR.
A 1-D frequency bandstop (notch) filter is known as the best instrument for narrow-banded noise elimination. Several different algorithms for notch filter design and implementation are known (see: Hamming R. W. “Digital filters”, Englewood Cliffs, N.J.: Prentice-Hall, 1985). In order to maximize the suppression of grid artifacts and to minimize image distortion, there are several filter transfer function parameters to tune, including cut frequency, bandwidth, attenuation level, and Gibbs event amplitudes.
The grid artifact suppression method proposed in U.S. Pat. No. 6,269,176, issued Jul. 31, 2001, inventors Barski et al., was based on 1-D adaptive gaussian blur filters design for use in the spatial domain. The principal disadvantage of that method is that Fourier transform of a gaussian filter is just a transfer function of a lowpass filter. It is very complicated to tune such a filter to the right cut frequency, i.e. grid peak frequency, and to compute this filter coefficients in the spatial domain. Moreover, the disadvantage of lowpass filter vs. a bandstop filter is that all frequencies higher than the one identified as the grid frequency in each specific gaussian filter are eliminated. This is the reason that such filters cause image blur, which in many cases may be unacceptable from a clinical point of view.
In the present invention a method of 1-D frequency bandstop filter is proposed as the method that best corresponds to the noise nature of the grid artifact, for the purpose of removing both grid line artifacts and Moiré patterns, which are very noticeable when a softcopy image display is resized. There were no prior efforts found in the patent record to use a frequency notch filter to suppress grid artifacts.